Digital testing of codec line cards is disclosed in Marsh et al, U.S. Pat. No. 5,596,322 issued Jan. 21, 1997 which describes Lucent Technologies Inc's T7531/35, 16-channel programmable codec. This codec includes a digital signal processor (DSP) which applies a digital calibration signal to the digital to analog (D/A) path. While this codec has the capability of synthesizing an analog termination impedance, it does not ascertain the actual impedance presented by the line to which it is connected and therefore the receive and transmit equalizers and other codec parameters are set to match only nominal, CCITT-specified values. Haughton U.S. Pat. No. 5,396,553 measures loop resistance (not impedance) to set the amount of side tone loss to be introduced by the codec. Lopresti U.S. Pat. No. 5,559,440 describes digital tests of the transmit and receive paths using a personal computer connected to the line circuit's PCM bus. A/D and D/A channel gains, return loss, terminal balance return loss, noise and distortion parameters are determined, but loop impedance is not conveniently ascertainable.
While the foregoing approaches are indeed quite useful, to achieve better performance it would be advantageous to be able to measure the actual impedance of the line and then set the receive and transmit equalizers and other codec parameters to match this value. A more exact match would eliminate the necessity of using inverse filtering to eliminate the effects of a mismatch. Moreover, a more exact match to the line impedance would also improve voice quality and facilitate the use of higher speed modems for which inverse filtering becomes increasingly more difficult. However, the ascertainment of line impedance has hitherto required more time than is compatible with call processing operations.